1. Why does state still show its old value after a setter?
Each render works with its own snapshot of state. Calling a setter requests another render; it does not change the value already captured by the current event handler. If you log that value immediately after calling its setter, the handler still sees the snapshot from its render.
Follow-up to practise: explain why repeated updates based on the previous value may need a functional updater. Write a counter example and predict the result before running it. [1]
2. When should you use an Effect?
Use an Effect to synchronise a component with an external system, such as a subscription or a browser API. A value that can be calculated from existing props or state usually belongs in rendering logic. Logic caused by a particular user action often belongs in that event handler.
Practice example: a search field and a list already exist in component state. Explain why storing a second copy of the filtered list can introduce unnecessary synchronisation work. Then discuss what changes if the results come from a server. [2]
3. Why do list items need stable keys?
Keys help React match sibling items between renders. Use a stable identity from the data when available. An array index is risky when items can be inserted, removed or reordered because a position does not consistently identify the same item.
Practice example: imagine editable rows in a shopping list. Delete the first row and explain why preserving each remaining row’s identity matters. Avoid generating a new random key during every render. [3]
4. How would you investigate a slow search screen?
Practice scenario: typing into a product search field feels delayed when the result list is large. Begin by clarifying whether the delay comes from input handling, rendering, network requests, or processing the response. Reproduce the issue with realistic data before choosing an optimisation.
A useful answer sets out an investigation: inspect the expensive work, isolate the smallest reproduction, propose a targeted change, and compare the result under the same conditions. Explain what evidence would make you change your initial hypothesis. Do not start by promising that adding memoisation everywhere will fix it.
5. How would you discuss a React feature you delivered?
Pick one feature and describe the user problem, component boundaries, data flow and error states. Explain one decision you made and an alternative you considered. Finish with how you checked the behaviour and what you would improve now.
For example, discuss a profile editor: where the draft lives, when it is saved, how validation errors appear, and what happens if saving fails. This is a practice outline; substitute your real project and avoid claiming results you cannot support.
A focused React practice checklist
Before a session, choose one question and answer it aloud in about a minute. Next, write a small working example. Finally, ask yourself what changes when the data is empty, a request fails, or the user repeats an action quickly.
For an early-career interview, prioritise clear explanations and correct small examples. For a senior discussion, add constraints, alternatives and how the decision affects maintenance. Use the job description to decide how much time to spend on testing, accessibility, routing or application architecture.
Continue your preparation with InterviewFM
Listen to interviewer–candidate conversations to practise the structure of a response, then pause and explain the same idea in your own words. Search the app for content relevant to your role, or generate a personalised interview from your role and experience. Keep a separate coding session for testing your understanding.
Frequently asked questions
Should I memorise React interview answers?
Use answers to understand the reasoning, then practise an example and a follow-up. Repeating a definition is less useful when the interviewer changes the scenario.
What should experienced React developers prepare?
Bring examples of architecture decisions, debugging, testing and collaboration. Explain constraints and trade-offs using work you actually did, alongside the framework fundamentals.
Technical references
Examples and practice scenarios are original exercises.
Listen. Learn. Practise your answer.
Explore interview conversations in InterviewFM and build a preparation routine around your role.